There's probably a technical limitation to that... but I actually thought about something:

Why not let the blue mage spellbook bar (the one you allocate the spells to) be used as an option for a hotbar?

Or maybe while on that bar, what you drag and drop to your hotbars is actually the spell slot and not the spell itself? So it would change depending on what's assigned to that slot in the blue mage book.

I fully agree that would be a huge QoL